1. Field of the Invention
The present invention relates to an improved system and method for enabling a node, such as a remote unit in a wireless network, to perform macroscopic selection and distribution of routes for routing data packets to other nodes in the network. More particularly, the present invention relates to a system and method for enabling nodes in an ad-hoc packet switched communications network to reduce the effects of signal fading by modifying request to send (RTS) and clear to send (CTS) messages to include additional unicast addressing fields comprising two sets of multiple unicast addresses representing the maximum number of data packet routes available per route entry in the node's routing table.
2. Description of the Related Art
Wireless communications networks, such as mobile wireless telephone networks, have become increasingly prevalent over the past decade. These wireless communications networks are commonly referred to as “cellular networks”, because the network infrastructure is arranged to divide the service area into a plurality of regions called “cells”.
Specifically, a terrestrial cellular network or other type of conventional specialized mobile radio (SMR) system includes a plurality of interconnected stationary base stations that are distributed geographically at designated locations throughout the service area. Each stationary base station includes one or more transceivers that are capable of transmitting and receiving electromagnetic signals, such as radio frequency (RF) communications signals, to and from user terminals, such as wireless telephones, located in its coverage area. The communications signals include, for example, voice data that has been modulated according to a desired modulation technique and transmitted as data packets. As can be appreciated by one skilled in the art, the transceiver and user terminals transmit and receive the data packets in multiplexed format, such as time-division multiple access (TDMA) format, code-division multiple access (CDMA) format, frequency-division multiple access (FDMA) format, orthogonal frequency division access (OFDM) or other suitable modulation formats, which enables a single transceiver at the base station to communicate simultaneously with several user terminals in its coverage area.
Each base station is also connected to one or more gateways that enable communication between the base station and other networks, such as the Internet and the public switched telephone network (PSTN). Accordingly, the base stations in the network enable the user terminals to communicate with each other, as well as with other destinations, such as telephony devices, in the PSTN.
Because each base station is stationary and can only handle a limited amount of communications signal traffic from the user terminals at any given time, the coverage area of a base station can vary depending on the amount of traffic that the base station is expected to experience. For example, the coverage area of a base station can be set to several kilometers in diameter in sparsely populated regions, such as rural regions having light wireless traffic, and can be set to less than a kilometer in diameter in densely populated regions, such as major metropolitan areas having heavy wireless traffic. The wireless communications network therefore must employ many stationary base stations in heavily populated metropolitan areas in order for the network to adequately service the user terminals in those regions.
As can be further appreciated by one skilled in the art, it is also common for a mobile user terminal to travel between different base station coverage areas during use, that is, during a single telephone call. When this occurs, the base station whose coverage area the user terminal is leaving must transfer or “handoff” the user terminal to the base station whose coverage area the user terminal is entering, so that the latter base station can become the base station via which the user terminal and network continue to communicate. In densely populated areas having many base stations with small coverage (often sectorized) areas, this handoff process may need to occur several times during a short period of time as the user terminal travels between the different cells and sectors. However, in regions such as high traffic commuting regions having an inadequate number of base stations or overlays or sectors, more user terminals are competing for access to a base station within their coverage area. Accordingly, the number of lost or dropped calls that may occur during the handoff process can be increased due to the lack of adequate base station accessibility and the break before make hard handoff process itself.
Selection and distribution of data packet routes within wireless communication systems, such as those described above, is known in the art. One such communication system employing selection and distribution is a Code Division Multiple Access (CDMA) communication system as described in Cellular System Remote Unit Base Station Compatibility Standard of the Electronic Industry Association/Telecommunications Industry Association Interim Standard 95A (IS-95A/B), which is incorporated by reference herein.
Many techniques have been developed using the circuit-switched cellular infrastructure to minimize data packet loss during handoff while also minimizing overhead necessary to successfully perform the handoff. For example, as described in IS-95A/B, as a remote unit moves to the edge of a cell, it may commence communication with an adjacent base station, while the current base station continues to handle the call. Both base stations then handle the call simultaneously. During such a scenario, the remote unit is said to be in soft handoff. Soft handoff provides diversity of forward traffic channels and reverse channel paths on the boundaries between base stations. It should also be noted that when the soft handoff occurs between sectors it is referred to as a “softer handoff”. Each base station involved in a particular soft handoff demodulates the traffic channel frames and provides the frames to a selector function. The selector function then selects the best frame from each of the active call legs and that frame is forwarded on to the rest of the communication network. This can also employ methods such as maximal ratio combining as is typically done in rake receivers or simple packet error detection methods. Likewise, the communication network provides frames to a distribution function that are to be transmitted to the remote unit. The distribution function distributes these frames to all base stations involved in soft handoff with the remote unit. Thus far this powerful technique has only been widely applied and integrated into CDMA based cellular systems such as those specified and proposed by the Third Generation Third Party Partnership Project (3GPP) Universal Mobile Telecommunications System (UMTS), 3GPP2 (CDMA 2000/IS-95C), IS-95 A/B (2G CDMA) and International Mobile Telecommunications (IMT) 2000.
Macro-diversity selection and distribution is used in cellular systems to allow mobile units to simultaneously communicate with multiple base stations, thus greatly increasing the chances that at least one of the paths will not experience any particular fade or possibly a shadowing. Mobile radios often can experience deep fades of 40 dB or more for milliseconds at a time. FIGS. 1A through 1C depict three independent Rayleigh fading paths of equal envelope power at a frequency of 2.45 GHz. The random occurrence of significant fading events is noteworthy. Fading behavior scales with frequency as a linear compression of the time axis as shown in FIG. 2A. Hence, for mobile radios, the fade rate for a given fade depth will increase linearly with platform speed whereas the mean fade duration will decrease nonlinearly with speed, as depicted in FIGS. 2B and 3, which is described in the publication by Jhong S. Lee and Leonard Miller entitled “CDMA Systems Engineering Handbook”, Artech House Publishers, 1998, and is herein incorporated by reference.
U.S. Pat. Nos. 6,226,283, 6,141,559, and 6,072,790 to Neumiller et al., the entire contents of each being incorporated herein by reference, disclose methods for providing macro-diversity selection and distribution in a peer-to-peer distributed manner where the SDU function was moved to the base transceiver stations. However, in hybrid multi-hop ad-hoc network systems that have fixed nodes to backhaul traffic to and from the Internet and the Public Switched Telephone Network (PSTN), a mobile node will use only one of the many routes it may have accumulated via routing protocols to its gateway. A mobile node may also send packets through other mobile nodes if the routing algorithm has no better choices.
Thus, for a fast moving mobile terminal in an ad-hoc network that has an established route, the signal can suffer from fading as those seen on interstate highways when mobile nodes/terminals are surrounded by a large amount of RF scattering clutter surfaces and reflectors. The call could then be dropped.
Accordingly, a need exists for a system and method for providing macro-diversity selection and distribution in ad-hoc communication networks to minimize the affects of fading.